The invention relates generally to the field of electric lamps and visual projection systems, and more particularly to high intensity discharge lamps employed for use in the visual projection systems.
High Intensity Discharge (HID) lamps are high-efficiency lamps that can generate large amounts of light from a relatively small source. These lamps are widely used in many applications, including highway and road lighting, lighting of large venues such as sports stadiums, floodlighting of buildings, shops, industrial buildings, and projectors, to name but a few. The term “HID lamp” is used to denote different kinds of lamps. These include mercury vapor lamps, metal halide lamps, and sodium lamps. Metal halide lamps, in particular, are widely used in areas that require a high level of brightness at relatively low cost. HID lamps differ from other lamps because their functioning environment requires operation at high-temperature and high pressure over a prolonged period of time. Also, due to their usage and cost, it is desirable that these HID lamps have relatively long useful lives and produce a consistent level of brightness and color of light. Though in principle the HID lamps can operate with either an alternating current (AC) supply or a direct-current (DC) supply; in practice, however, the lamps are usually driven via an AC supply.
Typical construction of an HID lamp includes a pair of electrodes enclosed within an arc tube with a pressurized gas. Light is generated by the hot gas or “plasma,” sometimes referred to as a “discharge” made by an electrical current that flows through the gas. The electrodes play a significant role in determining the amount of brightness of the light produced by the HID lamp. Electrode material is typically a refractory metal such as tungsten. The construction of the lead wire assembly includes a combination of one or more metals having a high melting point. Examples of materials used in the lead wire include tungsten, niobium, and molybdenum. During operation, current applied to the electrodes causes a decrease in resistance of the gas by creating a plasma discharge, permitting the flow of electrons across the gas medium and between the electrodes. This decrease in resistance causes the current to increase continuously. A driving circuit or ballast regulates the current and voltage applied to the electrodes.
The shortest distance of separation between the two electrodes positioned at opposite ends of the arc tube is called the arc length. This is the distance an arc jumps in the high-pressure gas medium to produce a discharge of light. The temperature of the electrode tip at the instant the arc appears increases substantially. Due to the decreasing resistance resulting from the arc, current increases and causes heating of the exposed electrode tip. This heating may, in fact, cause vaporization of the electrode tip, followed by recondensation of the electrode material, eventually forming a spike or extension at the tip. This change can result in reduced life of the HID lamp, a flicker in the emitted light (as the point of discharge changes with the tip geometry), a temporary change in the arc length, and a voltage variation across the electrodes. Flicker is primarily caused when the arc reattaches itself to the electrode at various spots. In projection systems, for example, this manifests itself as changes in intensity of light on projection systems due to occurrence of maximum intensity of light in spots not always at the focal point of lens assemblies in the projection systems. All of these effects are undesirable.
Currently existing techniques attempt to address the various effects by increasing the dimension of the electrodes at their tips. This results in a reduction in temperature of the electrode tip during arcing. However, the electrodes still undergo a change in geometry due to vapor transport of electrode material. The increased dimension of the electrode tips also lead to a less stable arc for reasons discussed earlier. Other existing solutions include control of the waveform used to drive the lamps. However, these have not fully addressed the problems or resolved the issue of flicker, useful life or control of the electrode tip geometry.
There is, therefore, a need for an improved approach to controlling an HID lamp that reduces the continuous change of electrode shape during operation of the lamp. There is a particular need for lamps of this type that exhibit reduced or little flickering of emitted light, and reduced voltage variation, with prolonged life.